Throughout the prosecution and the aftermath of the Civil War, Lincoln only used the Constitution when it became convenient for goals that he saw as more overriding. In fact, the whole premise of the Civil War was a usurption of the Constitution. Slavery, at the time, was written into the Constitution. In order to amend the Constitution, it would have taken three fourths of the states to ratify an amendment. Of course, this was never going to happen since the Southern states would vote in unison against changing the Constitution. At the time of Lincoln's inauguration, he planned on including several more states into the Union and each would likely vote to ratify an end to slavery. Before this could happen Southern states began one by one to secede.
The American Civil War (1861–65) began because leaders of Southern states were unhappy with the outcome of the 1860 presidential election, which was won by Abraham Lincoln (1809–1865). Fearful of losing their economic system, which was based on agriculture and dependent on slave labor, the Southern states began to act on their promise to secede (withdraw) from the United States (called the Union) and form their own nation. South Carolina was the first to secede, in December 1860. Five more states—Mississippi, Florida, Alabama, Georgia, and Louisiana—followed in January 1861. When representatives from the six states met the next month in Montgomery, Alabama, they established the Confederate States of America (commonly called the Confederacy) and elected Jefferson Davis (1808–1889) as president. Two days before Lincoln's inauguration...
All of this was in fact perfectly within their rights as states. Lincoln forced the Civil War on the South even though they did everything within the confines of the Constitution.
In 1862, Abraham Lincoln not only declared martial law but suspended habeas corpus.
Along with a declaring martial law, President Abraham Lincoln ordered the suspension of the constitutionally protected right to writs of habeas corpus in 1861, shortly after the start of the American Civil War. At the time, the suspension applied only in Maryland and parts of the Midwestern states.
The Supreme Court eventually ruled that Lincoln was acting outside the confines of the Constitution in suspending habeas corpus though this ruling came after the war ended.
Finally, the 14th amendment
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
which effectively ended slavery was also ratified without the proper seventy five percent of the states ratifying the amendment. Lincoln faced roughly the same problem as he faced in attempting to end slavery prior to the war. The Southern states were never going to go for such an amendment. The first confrontation lead to war. After the war, Lincoln simply ignored the guidelines laid out in order to add an amendment. Rather than following the Constitution, Lincoln simply imposed the 14th amendment on the defeated Southern states who were then in no position to argue.
Besides the bouts with a lack of Constitutional clarity, Lincoln also faced an ever more tenuous war effort. Here is how the 1864 Democratic Party platform...the platform of his opponents.
Resolved, That in the future, as in the past, we will adhere with unswerving fidelity to the Union under the Constitution as the only solid foundation of our strength, security, and happiness as a people, and as a framework of government equally conducive to the welfare and prosperity of all the States, both Northern and Southern.
Resolved, That this convention does explicitly declare, as the sense of the American people, that after four years of failure to restore the Union by the experiment of war, during which, under the pretense of a military necessity of war-power higher than the Constitution, the Constitution itself has been disregarded in every part, and public liberty and private right alike trodden down, and the material prosperity of the country essentially impaired, justice, humanity, liberty, and the public welfare demand that immediate efforts be made for a cessation of hostilities, with a view of an ultimate convention of the States, or other peaceable means, to the end that, at the earliest practicable moment, peace may be restored on the basis of the Federal Union of the States.
Before finally attaining victory, Lincoln faced many dark days in which victory was not only not inevitable but frankly never in sight.
So, why did Lincoln subvert the Constitution and how did he get through all those dark days? Of course, only Abraham Lincoln himself really knows. I firmly believe though that Abraham Lincoln knew in his core that slavery was the great evil of its time. He knew that it must be confronted and that's what brought him into politics to begin with. He knew that even the Constitution itself could not be an unwitting accomplice in perpetuating this evil. Even in his darkest days, he knew that he was on the side of good against evil and that's what got him through. It was this moral clarity that got him through, and it's that moral clarity that has lionized his legacy. It was that moral clarity that allowed him to confront the evil of slavery without flinching. It's that sort of moral clarity that we need more of now.